Process of increasing the permeability of oriented silicon steels



Patented June 3, 1952 PROCESS OF INCREASING THE PERMEA BILITY OF ORIENTED SILICON STEELS West Middletown, and Joseph E. Heck, Monroe, Ohio, assignors to Armco Steel Corporation, Middletown, Ohio, a

Martin F. Littmann,

corporation of Ohio No Drawing. Application October 21, 1948,

Serial No. 55,825

Claims. (01. 14s-12.s

Our invention relates to the production of silicon steel for magnetic uses, the steel being characterized by a high degree of preferred orientation, and being highly directional in properties. More particularly our invention relates to the production of silicon steel having that type of crystal orientation in the rolling direction known as [100] (011) or cube-on-edge, and the derivatives of such orientation.

The fundamental object of our invention is the provision of a procedure whereby enhanced degrees of preferred orientation may be obtained in silicon steels, and the provision of certain preliminary steps which are effective in enhancing the degree of preferred orientation produced by other and subsequent steps, all as Will hereinafter be set forth.

It is thus an object of our invention to provide a series of steps which, practiced as preliminary to the steps comprising any of the known processes of making highly oriented silicon steel, will produce a marked and useful change in the perfection of the orientation so produced.

These and other objects of the invention, which will be set forth hereinafter or will be apparent to one skilled in the art upon reading these specifications, we accomplish by that series of process steps and treatments of which we shall now describe certain exemplary embodiments.

The material to which the invention relates is silicon steel in general having a silicon content of about 2.5 to 4.0%. A typical but non-limiting analysis for such a steel after the final anneal is as follows:

Hitherto in the art, various processes for making oriented silicon steels have been known.

iThese comprise in general a heavy cold rolling reduction followed by a high temperature final anneal for developing the ultimate magnetic characteristics. 'Certain processes comprise two or more cold rolling reductions with an intermediate open anneal at a temperature around 1500 to 1850 F., the high temperature anneal following the last cold rolling reduction. In the usual practice, the silicon steel as produced by the open hearth or other suitable melting technique is cast into ingots which are heated in the soaking pits and rolled. into slabs on a blooming or slabbing mill. The slabs are then heated in a slab furnace and rolled to a desired hot roll gauge in a continuous hot rolling mill comprising roughing and finishing stands. The hot rolled steel then is the starting material for any of those routings comprising cold rolling and heat treatment steps which have been generally characterized above. The value of a heat treatment subsequent to hot rolling and prior to cold rolling has hitherto been recognized; and this heat treatment may either be in the nature of a box anneal, or it may be a continuous heat treatment at 1400 to 2000 F. The material is pickled prior to cold rolling to remove surface oxides.

Since for most uses a low carbon, low core loss material is desired, it is usual to include a decarburizing treatment in the routing. While other treatments are available, this may be and preferably is an open anneal in a decarburizing gas at about 1500 F. for a brief time varying with the thickness of the material. In many processes such a decarburizing treatment will follow all cold rolling treatments, and be followed in its turn by the high temperature final anneal, usually a box anneal in dry hydrogen at around 2050 F. or higher.

It has also long been recognized that the number of cold rolling treatments, the correlation of the specific reductions produced in each, and the temperatures of the intermediate and final anneals exert a controlling effect upon the degree of preferred orientation produced. These variables will not herein be specifically discussed nor their correlation pointed out for the reason that the new steps herein taught are applicable to any and all of the known processes which by themselves result in the production of the so-called oriented silicon steels. We have found that the new steps herein taught, applied to any of these processes will result in a useful enhancement of the'perfection of the preferred orientation and hence a higher permeability in the rolling direction.

The permeabilities referred to herein are permeabilities measured in a direction parallel to the rolling direction in stress free material at a field strength of 10 oersteds, using a test density of the steel of 7.65. The maximum possible permeability, i. e. the permeability of a single crystal measured in the best direction is believed to lie between 1900 and 1950 for steel of 3.2% silicon content. These values are not attained in practice, and various prior art processes have, neglecting sporadic variations, produced permeabilities falling short of these ideal values. In processes including the steps of this invention, permeabilities up to 1835 have been obtained consistently. This'may be compared. to an average permeability of 1450 in the same material under the same conditions but tested in a direction. transverse to the rolling direction. and to similar values for non-oriented steel of the same analysis tested parallel to the rolling direction. The practice of the steps herein outlinedcanbe depended upon to add about 100 to the permeability values: hitherto produced by specific routings disclosed in prior art patents, and with certain routings have produced improvements as: high as 280..

Another commonly used basis for comparison of permeabilities is to measure the permeabilities at a constant induction of 16 kilo'g-aus'se's. On this basis an improvement from 1660 (a common permeability in oriented silicon. steel prior to. our invention) to 1835, measured at constant magnetizing force of 10 oersteds, is equivalent-to an improvement from 290.0. to;'23,0010 at constant induction. This is a very important. improve ment since it means that the excitingcurrentgin a transformer operating at. 16 kilogausses. will be nearly eight tirnes as high when constructed of material having a permeability of 1660 at. 10

oersteds as for atransformer built with the: same Weight. of core from material having apermeability of I835; Thus a small increase the permeability at 10 oer-steels will. result. in a large decrease in. the exciting current-required at the normal operating induction. Suchan improvement in exciting current of importaMeinpermitting substantial savingsin the weight of cores and. windings, provided it accompanied. by a core loss per un-itpweight atya given. induction such that, (asv may be. the. case with materials made in accordance herewith) the total energy loss of the lighter core will be comparable to that of. the heavier core made of materialspreviously available.

Hitherto in. the: development: of processes for producing high permeability silicon ferrite sheets or strip, attention; has: been ifocusedprimaril y upon the amounts ofcold reduction in theseveral stages and the temperatures; of the; intermediate andfi'nal anneals. Whilerelatively high permeabilities have been seoured;jthe products in general was, characterized: by irregularities;variations in permeability as between parts: of individual coils produc'ed, and non -reducibility of results under many. circumstances; We have found that there are variables affecting the; orientation of the product which have. notahithefrto been understood or considered. These. variables are of such importance in the production of highv degrees of orientation that. attention. to them in accordance; with, the teachings; herein can be depended upon. to produce. marked and useful increases in permeability; ashes; beamindicated. Moreover-,attention to these: variables tends very greatly to minimize the irregulari ties mentioned above.

There has hitherto been. some, suggestionin the art that the hot rolling temperature, might affect the ultimatepermeability. Wahavefound that the actual temperature of hot; rolling, including the rate of cooling during and: subsequent to hot rolling, and specific temperaturesat the conclusion of the hot reduction, .whileaofa'some importar'ioe,,are,relatively minor in effect; We have found. that: an. especially important; variable hitherto unrecognized lies in the temperature attained by the material prior to hot strip rolling, and especially in the form of slabs suitable for hot rolling.

In accordance with our invention it is our practice to heat the slabs to the highest possible temperature without producing that intergranular disintegration known as burning. The heating to the highest possible temperature is most conveniently done in the slab furnace ahead of the continuous hot mill; but it is possible to approximate the eflect by separately an- Healing the slabs at high temperatures, say, temperatur'es near 2500 F., and subsequently rolling them at lower-temperatures, say, at temperatures near 2300" F. After the separate annealing. the slabs may be cooled, and then reheated to the lower temperatures in the usual slab furnaee'.

The burning to which we have referred is a progressive interg-ranular disintegration probably due to; oxidation or to the migration of intergrahular substances or both, and usually occurs in the neighborhood of 2600 F. in an oxidizing atmosphere. While somewhat higher tempera tures might safely be employed in a separate annealing furnace in which'a non-oxidizin atmosphere can be maintained, we; usually prefer to carry on the slab heating in the ordinary slab furnaces which are a part of the; hot continuous mill, installation. Since the measurement of these high temperatures presents some difificulties and Will usually be accomplished by the use of pyrometers located either at the exit of, the slab furnace or between the roughing and finishing stands of the hot continuous mil-1 calculations of actual temperatures will normally be required, although it is possible; to use a radiation pyrometer inside the slab furnace. It will be found preferableto operate with a margin of safety under the burning temperature. We have found that excellent results may be attained by heating the slabs to temperatures lying 'SllbStEtll-r tially between 2360" F. and about 2550 F. with the emphasis. on the higher temperatures, for better results. The time during which the slabs are held at the highest temperature isv not of maximum importance; but it must be remembered that pyrometer measurements are usually measurements of surface temperatures. so that care, should be taken to make. sure that the interior portions. of the slabs are brought up to the desired temperature. This can most conveniently be accomplished by holding'the slab at temperature for a period of time. For example, satisfactory results are secured in heating slabs 3-in. thick to a temperature of about 2550 F.

iii theiurnace temperature is above that value and theslabs are permitted to-remain. in theslab furnace-for at least aboutBO-minutes after their surfaces have attained temperatures-of about 2300 'F;

We prefer to heat our slabs to temperatures within the above noted range, preferably to temperatures around or above 2500 F.

A typical but non-limitingrouting for oriented siliconsteel including the principles, of thisinvention, may be set forth as follows:

Heat. ingots to about 2350. F.

Roll, into slabs of an appropriate thickness which will usually be 3 in. to 6in., but may be varied depending upon the size of coil desired.

Heat slabs in slab furnace to the maximum.

- temperature possible without burning.

Hot roll the slabs as rapidly as possible to a thickness 5.5 to 9 times the final thickness.

Open anneal the hot rolled material at a temperature between 1400" and 2000 F. and pickle.

Cold roll the annealed material to a thickness between 1.3 and 2.5 times the final thickness.

Open anneal the cold rolled material at a temperature between 1400 and 1850 F.

Cold roll to final gauge.

Open anneal preferably in a decarburizing atmosphere, as in wet reducing gas, at about 1500 F.

Box anneal the product in dry hydrogen at 2000 F. or above, preferably at around 2100" F.

In such an exemplary process, the raising of the slab temperature from a temperature of about 2100 F. to the highest attainable temperature without burning, say, a temperature of about 2550 F., all other conditions remaining the same, will produce an increase of 100 to 280 in permeability measured at 10 oersteds. The degree of improvement will vary depending upon the values chosen for the processing variables within the exemplaryranges.

We have also investigated other variables connected with treatment prior to cold rolling. The temperature to which the ingots are heated prior to slabbing has some effect, and increase in ingot temperature will produce a relatively small increase in ultimate permeability in the rolling direction. Therefore, we prefer to heat the inacts to about 2300 F. or above.

The actual temperature at which the hot rolling is carried on is not of maximum importance in itself. It will be understood that the temperature at which the hot rolling is completed may be determined to a considerable extent by the initial slab temperature, so that a high slab temperature usually makes for hot rolling at higher temperatures. The most important processing variable at and prior to the hot rolling stage is the attainment by the slabs of a high temperature as taught herein, not the temperature at which the hot rolling itself is conducted or finished. However, we prefer to hot roll the slabs to gauge promptly and with no more cooling than is inherent in the operation of the particular apparatus at hand.

The heating of slabs to the temperatures herein taught appears to increase their grain size, and to affect the result of subsequent steps in any routing for the production of oriented silicon steel to the extent of producing a marked and valuable increase in the ultimate permeability and directional characteristics.

Modifications maybe made in our invention without departing from the spirit of it. Havin thus described our invention in certain exemplary embodiments, what we claim as new and desire to secure by Letters Patent is:

1. A process of producing highly oriented silicon steel for magnetic purposes, said steel containing substantially 2.5 to 4% silicon, including as steps an initial hot rolling of the steel from slabs followed by a cold rolling treatment and a final high temperature anneal, which process is characterized by the step of heating the said slab prior to hot rolling to a temperature of substantially 2500 F.

2. A process of producing highly oriented silicon steel for magnetic purposes having as steps a heating of silicon steel ingots, said ingots containing substantially 2.5 to 4% silicon, a tormation of said ingots into slabs. and a 'hot tOllins oi the steel from said slabs followed by at least one cold rolling treatment and a final high temperature box anneal, which process is characterized by the steps of heating said ingots to a temperature of at least substantially 2300 F. preparatory to slabbing, and heating said slabs prior to hot rolling to a temperature above about 2300" and up to about 2550 F.

3. A process of producing highly oriented silicon steel for magnetic purposes having as steps an initial hot rolling of the steel containing substantially 2.5 to 4% silicon from slabs followed by at least one cold rolling treatment and a final high temperature anneal, which process is characterized by the step of heating said slabs prior to hot rolling to temperatures above about 2300 and up to about 2550 F.

4. A process of producing highly oriented silicon steel for magnetic purposes, said steel containing substantially 2.5% to 4% silicon, having as steps an initial hot rolling of the steel from slabs followed by two cold rolling stages and an intermediate anneal, which process is characterized by the step of heating the said slabs prior to hot rolling to a temperature above about 2300 F. and up to about 2550 F.

5. The process claimed in claim 4 includin the steps of forming said slabs from ingots and heating said ingots prior to slabbing to a temperature of at least substantially 2300" F.

6. The process claimed in claim 4 in which said intermediate anneal is an open anneal at a temperature substantially between 1400" and 1850 F.

7. The process claimed in claim 4 in which the second cold rolling step is followed by a decarburizing treatment including an open anneal, and by a final box anneal.

8. The process claimed in claim 4 in which said intermediate anneal is an open anneal at a temperature substantially between 1400 and 1850 F., in which the second cold rolling step is followed by an open anneal and by a final box anneal in dry hydrogen at a temperature of substantially 2100 F.

9. The process of claim 8 in which the steel is open annealed at a temperature of substantially 1400 to 2000 F. between the hot rolling and the first cold rolling treatments.

10. A process of producing highly oriented silicon steel for magnetic purposes, said steel containing substantially 2.5 to 4% silicon, having as steps an initial hot rolling of the steel from slabs followed by at least one cold rolling treatment and a final high temperature anneal, which process is characterized by the steps of heating said slabs prior to hot rolling to temperatures above about 2300 and up to about 2550 F., cooling said slabs and reheating them to a temperature appropriate for hot rolling.

MARTIN F. LITTMANN. JOSEPH E. HECK.

REFERENCES CITED UNITED STATES PATENTS Name Date Goss June 22, 1937 OTHER REFERENCES The Alloys of Iron and Silicon," by Greine et al.,- McGraw-Hill Book 00., N. Y., 1933, p. 367.

Number 

1. A PROCESS OF PRODUCING HIGHLY ORIENTED SILICON STEEL FOR MAGNETIC PURPOSES, SAID STEEL CONTAINING SUBSTANTIALLY 2.3 TO 4% SILICON, INCLUDING AS STEPS AN INITIAL HOT ROLLING OF THE STEEL FROM SLABS FOLLOWED BY A COLD ROLLING TREATMENT AND A FINAL HIGH TEMPERATURE ANNEAL, WHICH PROCESS IS CHARACTERIZED BY THE STEP OFHEATING THE SAID SLAB PRIOR TO HOT ROLLING TO A TEMPERATURE OF SUBTANTIALLY 2500* F. 